The present invention relates generally to semiconductor memory devices and more particularly to flash electrically erasable programmable read only memory (EEPROM) cells that utilize the phenomenon of hot electron injection to trap charge within a trapping dielectric material within the gate.
Memory devices for non-volatile storage of information are currently in widespread use today, being used in a myriad of applications. A few examples of non-volatile semiconductor memory include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM) and flash EEPROM.
Semiconductor ROM devices, however, suffer from the disadvantage of not being electrically programmable memory devices. The programming of a ROM occurs during one of the steps of manufacture using special masks containing the data to be stored. Thus, the entire contents of a ROM must be determined before manufacture. In addition, because ROM devices are programmed during manufacture, the time delay before the finished product is available could be six weeks or more. The advantage, however, of using ROM for data storage is the low cost per device. However, the penalty is the inability to change the data once the ROM has been manufactured. If mistakes in the data programming are found they are typically very costly to correct. Any ROM inventory that exists having incorrect data programming is instantly obsolete and probably cannot be used. In addition, extensive time delays are incurred because new masks must first be generated from scratch and the entire manufacturing process repeated, at least from the ROM programming mask step. Also, the cost savings in the use of ROM memories only exist if large quantities of the ROM are produced.
Moving to EPROM semiconductor devices eliminates the necessity of mask programming the data but the complexity of the process increases drastically. In addition, the die size is larger due to the addition of programming circuitry and there are more processing and testing steps involved in the manufacture of these types of memory devices. An advantage of EPROMs is that they are electrically programmed, but for erasing, EPROMs require exposure to ultraviolet (UV) light. EPROM dice are placed in packages with windows transparent to UV light to allow each die to be exposed for erasing, which must be performed before the device can be programmed. A major drawback to these devices is that they lack the ability to be electrically erased. In many circuit designs it is desirable to have a non-volatile memory device that can be erased and reprogrammed in-circuit, without the need to remove the device for erasing and reprogramming.
Semiconductor EEPROM devices also involve more complex processing and testing procedures than ROM, but have the advantage of electrical programming and erasing. Using EEPROM devices in circuitry permits in-circuit erasing and reprogramming of the device, a feat not possible with conventional EPROM memory. Flash EEPROMs are similar to EEPROMs in that memory cells can be programmed (i.e., written) and erased electrically but with the additional ability of erasing all memory cells at once, hence the term flash EEPROM. The disadvantage of flash EEPROM is that it is very difficult and expensive to manufacture and produce.
The widespread use of EEPROM semiconductor memory has prompted much research focusing on constructing better memory cells. Active areas of research have focused on developing a memory cell that has improved performance characteristics such as shorter programming times, utilizing lower voltages for programming and reading, longer data retention times, shorter erase times and smaller physical dimensions. One such area of research involves a memory cell that has an insulated gate. The following prior art reference is related to this area.
U.S. Pat. No. 4,173,766, issued to Hayes, teaches a metal nitride oxide semiconductor (MNOS) constructed with an insulated gate having a bottom silicon dioxide layer and a top nitride layer. A conductive gate electrode, such as polycrystalline silicon or metal, is placed on top of the nitride layer. A major disadvantage of this device is the difficulty in using it to construct a flash EEPROM. A consequence of using an oxide-nitride structure as opposed to an oxide-nitride-oxide structure is that during programming the charge gets distributed across the entire nitride layer. The absence of the top oxide layer lowers the ability to control where the charge is stored in the nitride layer.
Further, in the memory cell disclosed in Hayes, the nitride layer is typically 350 Angstroms thick. A thick nitride layer is required in Hayes"" device in order to achieve sufficient charge retention. Due to the thick nitride layer, very high vertical voltages are needed for erasing. The relatively thick nitride layer causes the distribution of charge, i.e., the charge trapping region, to be very wide and a wider charge trapping region makes erasing the cell via the drain extremely difficult if not impossible. Thus, the memory cell taught by Hayes must have a thick nitride layer for charge retention purposes but at the expense of making it extremely difficult to erase the device via the drain, thus making the device impractical for flash EEPROM applications.
To erase the memory cell of Hayes, the electrons previously trapped in the nitride must be neutralized either by moving electrons out of the nitride or by transferring holes into the nitride. Hayes teaches an erase mode for his memory cell whereby the information stored on the nitride is erased by grounding the gate and applying a sufficient potential to the drain to cause avalanche breakdown. Avalanche breakdown involves hot hole injection into the nitride in contrast to electron injection. Avalanche breakdown, however, requires relatively high voltages and high currents for the phenomenon to occur. To lower the avalanche breakdown voltage, a heavily doped impurity is implanted into the channel between the source and the drain.
The hot holes are generated and caused to surmount the hole potential barrier of the bottom oxide and recombine with the electrons in the nitride. This mechanism, however, is very complex and it is difficult to construct memory devices that work in this manner. Another disadvantage of using hot hole injection for erase is that since the PN junction between the drain and the channel is in breakdown, very large currents are generated that are difficult to control. Further, the number of program/erase cycles that the memory cell can sustain is limited because the breakdown damages the junction area. The damage is caused by the very high local temperatures generated in the vicinity of the junction when it is in breakdown.
In addition, it is impractical to use the memory device of Hayes in a flash memory array architecture. The huge currents generated during erase using avalanche breakdown would cause significant voltage (i.e., IR), drops along the bit line associated with the memory cell in breakdown.
Another well known technique of erasing is to inject holes from the gate into the nitride layer. This mechanism, however, is very complex and difficult to control due to the higher mobility of holes versus electrons in the nitride. With elevated temperatures, the higher mobility of holes causes a large loss of charge retention and consequently lower threshold voltage deltas. Deep depletion phenomena create the need for a companion serial device to control the programming/erase process.
U.S. Pat. No. 5,168,334, issued to Mitchell et al., teaches a single transistor EEPROM memory cell. Mitchell, however, teaches an oxide-nitride-oxide (ONO) EEPROM memory cell wherein oxide-nitride-oxide layers are formed above the channel area and between the bit lines for providing isolation between overlying polysilicon word lines. The nitride layer retains charge to program the memory cell.
Although the memory device of Mitchell includes a top oxide layer, it is not very well suited for flash EEPROM applications. This is due to the very wide charge trapping region that must be programmed in order to achieve a sufficient delta in the threshold voltage between programming and reading. The Mitchell device is programmed and read in the forward direction. Since reading in the forward direction is less effective than reading in the reverse direction, the charge trapping region must be wider by default in order to distinguish between the programmed and unprogrammed states. A wider charge trapping region, however, makes the memory device very difficult to erase, thus making this device inefficient for flash EEPROM applications.
A single transistor ONO EEPROM device is disclosed in the technical article entitled xe2x80x9cA True Single-Transistor Oxide-Nitride-Oxide EEPROM Device,xe2x80x9d T. Y. Chan, K. K. Young and Chenming Hu, IEEE Electron Device Letters, March 1987. The memory cell is programmed by hot electron injection and the injected charges are stored in the oxide-nitride-oxide (ONO) layer of the device. This article teaches programming and reading in the forward direction. Thus, as in Mitchell, a wider charge trapping region is required to achieve a sufficiently large difference in threshold voltages between programming and reading. This, however, makes it much more difficult to erase the device.
In accordance with the present invention, a flash electrically erasable programmable read only memory (EEPROM) and method of programming, reading and erasing the same are provided. In one embodiment, the flash EEPROM memory cell is constructed having a charge trapping dielectric layer sandwiched between two silicon dioxide layers. The non conducting dielectric layer traps electrical charge and the two layers of silicon dioxide act as electrical insulators. A conducting gate layer is placed over the upper silicon dioxide layer.
A novel aspect of the memory device is that while it is programmed in the conventional manner, using hot electron programming, it is read in a direction opposite that of programming. Each cell is programmed conventionally by applying programming voltages to the gate and the drain while the source is grounded. Hot electrons are accelerated sufficiently to be injected into a region of the trapping dielectric layer near the drain. The device, however, is read in the opposite direction from which it was written, meaning voltages are applied to the gate and the source while the drain is grounded.
Reading in the reverse direction is most effective when relatively low gate voltages are used. A benefit of utilizing relatively low gate voltages in combination with reading in the reverse direction is that the potential across the portion of the channel beneath the trapped charge region is significantly reduced. A relatively small programming region or charge trapping region is possible due to the lower channel potential. This permits much faster programming times because the effect of the charge trapped in the localized trapping region is amplified. Programming times are reduced while the delta in threshold voltage between the programmed versus unprogrammed states remain the same. Another major benefit is that the erase mechanism of the memory cell is greatly enhanced. The memory cell can be erased by applying suitable erase voltages to the gate and the drain so as to cause electrons to be removed from the charge trapping region of the nitride layer. Electrons move from the nitride through the bottom oxide layer to the drain. Another benefit includes reduced wearout from cycling thus increasing device longevity. An effect of reading in the reverse direction is that a much higher threshold voltage for the same amount of programming is possible. Thus, to achieve a sufficient delta in the threshold voltage between the programmed and unprogrammed states of the memory cell, a much smaller region of trapped charge is required when the cell is read in the reverse direction than when the cell is read in the forward direction.
The erase mechanism is enhanced when the charge trapping region is made as narrow as possible. Programming in the forward direction and reading in the reverse direction permit limiting the width of the charge trapping region to a narrow region near the drain. This allows for much more efficient erasing of the memory cell.
Further, utilizing a thinner silicon nitride charge trapping layer than that disclosed in the prior art helps to confine the charge trapping region to a laterally narrower region near the drain. Further, the thinner top and bottom oxide sandwiching the nitride layer helps in retention of the trapped charge.
In addition, unlike prior art floating gate flash EEPROM memory cells, the bottom and top oxide thickness can be scaled due to the deep trapping levels that function to increase the potential barrier for direct tunneling. Since the electron trapping levels are so deep, thinner bottom and top oxides can be used without compromising charge retention.
Another benefit of localized charge trapping is that during erase, the region of the nitride away from the drain does not experience deep depletion since the erase occurs near the drain only. The final threshold of the cell after erasing is self limited by the device structure itself. This is in direct contrast to conventional single transistor floating gate flash memory cells which are plagued with deep depletion problems. To overcome these problems, manufacturers include complex circuitry to control the erase process in order to prevent or recover from deep depletion.
Another approach previously employed in the prior art to solve the deep depletion problem was to design the floating gate flash memory cell using a split gate design forming multiple transistors per cell. The split gate or double transistor constructions were necessary because the information carrying transistor, i.e., the floating gate transistor, potentially could be over-erased. An over-erase condition caused the threshold voltage of the cell to go too low. The second transistor, acting as a control transistor, prevented the floating gate transistor from being over-erased.